Devices for the protection of over tensioning in spring motors for window shades and other devices have been in use for many years. One common approach to this problem is found in various forms in U.S. Pat. Nos. 350,656 to Brooks, 823,452 to Vickery, 873,438 to John, and 982,444 to Smith. Each of these patents disclose some form of a nut or other internally threaded element which is movable along a threaded shaft in response to the rotation of the shaft. A spring is fixed to the shaft and is tensioned or unwound with the rotation thereof in a given direction. The nut is positioned on the shaft so that it will engage a stop at a predetermined point when the desired number of turns of the shaft is reached. This predetermined point is made to correspond to the maximum tension which can be imposed on the spring motor without damage. Once the designated number of rotations is reached, further rotation of the shaft is prevented by engagement of the nut or other movable member with the stop.
Another approach is found in U.S. Pat. No. 1,052,689 to Pitman. In this patent, an elongated shaft is provided having a threaded portion at one end and an attachment means at the other end. A spring, acting as the motor for the window shade, is secured to the attachment means of the elongated shaft. A drum having internal threads is disposed along the threaded portion of the shaft and movable axially therealong in response to rotation of the shaft. The other end of the spring is attached to the drum, and thus, movement of the drum in one direction will cause the spring to tension and in the other direction to unwind. Means are provided to engage the drum and prevents its axial movement after a desired number of rotations of the shaft so as to provide a fixed stop to prevent the spring motor from being tensioned beyond a predetermined amount.
One limitation of each of the devices described above is that they are rather bulky in configuration and may not be readily adapted to the streamlined tubes which house modern day window shade devices. In addition, each of these over tensioning means provides a fixed stop which restricts the shaft from any further rotation in the direction in which the spring is tensioned. It is contemplated that this could present a practical problem in the use of window shades embodying such devices, particularly by those not familiar with their operation. Once the point where the stop engages is reached, the user could inadvertently pull further on the shade and create severe damage to the entire window shade assembly.
A solution to this problem is disclosed in U.S. patent application Ser. Nos. 396,649 filed July 9, 1982 and 401,832 filed July 26, 1982, both entitled Window Shade Clutch Assembly, and both assigned to the same assignee as this invention. These inventions each disclose a clutch assembly having clutch means operable to limit the amount of tensioning of the spring motor without providing a fixed stop. The clutch assemblies of such inventions are adapted for use with window shades having a rotatable housing, a fixed, threaded shaft and a spring motor attached at one end to the fixed shaft. The clutch assemblies generally include sleeve means, attached to the spring motor, which is adapted to move axially along the fixed shaft in a first and second direction. Connecting means are provided to connect the sleeve means to the housing for rotation therewith, which rotation causes the sleeve means to move axially along the fixed shaft. Movement of the sleeve means in one axial direction along the shaft, in response to rotation of the housing in a first direction, causes increased tensioning of the spring motor.
In order to prevent over-tensioning of the spring motor, clutch means is provided in each of the inventions which comprises a locking element formed in the sleeve means and a mating locking element formed in the connecting means. At a predetermined point of axial movement of the sleeve means along the shaft, the clutch means operates to disengage the locking element of the sleeve means from the mating locking element of the connecting means for up to one revolution of the housing in such first direction. Immediately thereafter, the fully tensioned spring motor which is attached to the sleeve means, operates to rotate the sleeve means in the opposite direction of such first direction for re-engagement with the connecting means. Further rotation of the housing in the first direction results in alternating disengagement and engagement of the sleeve means locking element with the locking element of the connecting means, which prevents additional tensioning of the spring motor while allowing the housing to continue to be rotated in such first direction.
The spring motors utilized in window shades to produce satisfactory performance impose a relatively high amount of torque on the sleeve means and connecting means of such clutch assemblies, particularly in re-engaging the sleeve means with the connecting means once the clutch means operates to disengage them. In the single or half revolution of the sleeve means which occurs before it re-engages the connecting means, the sleeve means builds up a significant amount of momentum due to the relatively high torsional force imposed through its connection to the spring motor. Therefore, the locking element of the sleeve means contacts the locking element of the connecting means with a relatively high impact force. Repeated contact between such locking elements at such high impact levels can produce wear of the locking elements of these prior designs unless hardened materials are used in their fabrication such as glass-filled Nylon. These materials add to the cost of the window shade.